Okla. Stat. tit. 12A, § 3-105
Oklahoma Code Comment
This Section is a combination of several prerevision sections. In relation to subsection (b) (pre-revision sub section 3-306(c) ) , and the defenses of conditional issuance and issuance for a special purpose, the parol evidence rule, operable through Section 3-117 , may not preclude evidence of a condition precedent to the enforceability of the instrument, because the evidence goes to whether there is a contract, rather than to contradict the contract terms. Cf Federal Nat'l Bank & Trust Co. of Shawnee v. Shanon Drilling 762 P. 2d 928 (Okla. 1988). However, a provision in the instrument, particularly a note, such as, "No conditions to the effectiveness of this instrument not contained herein, and no special purpose or purposes behind its issuance, exist," could operate to protect against an assertion of an agreement beyond the instrument creating such a condition or involving a special purpose. Note, however, that the parol evidence rule may exclude a condition subsequent that is contradictory to the terms of the instrument. See Jones v. Sageeyah Devel., Ltd. 833 P.2d 1235 (Okla. 1992).